Rhyolite is not a granite: rhyolites have a similar chemical composition to granite but a different fabric and texture as they formed in a different way. Rhyolites are essentially the extrusive equivalents to granites with lavas cooling relatively quickly on the Earth’s surface.
In Northern Ireland, rhyolites mainly occur in County Antrim (Tardree and Sandy Braes) with small occurrences in County Down. The rhyolite formed as part of the volcanic activity that formed the Tertiary Antrim Basalts 60 million years ago and erupted during a lull in volcanic activity in the area. Rhyolite is used as building stone throughout Co. Antrim as rubble stone, quoins and dressings. The stone is pale-coloured and comprises a fine-grained ash-like matrix with larger crystals (phenocrysts) of quartz and rare plagiocalse feldspar.
The rhyolite weathers most commonly by scaling. On quoin stones, once the tooled surface is breached, scaling proceeds relatively rapidly with deterioration of the matrix and detachment of quartz grains.